A novel type of serotonergic binding site was recently described in rat and pig choroid plexus. The rat binding site has been localized to the epithelial cell layer of the choroid plexus. The rat choroid plexus site is present at a density ten fold higher than the density of any other serotonergic site in the brain, which makes this a particularly attractive system for biochemical and cell biological analysis. We plan to extend our initial characterization of this new serotonergic site in several directions. An autoradiographic survey of the rat brain will reveal if other areas of the brain contain the novel 125I-LSD binding site found in choroid plexus. Human choroid plexus samples from autopsy brains will be studied to determine if they contain this new serotonergic site. Localization of the rat choroid plexus site will be extended to the subcellular level to determine if the site is located on the apical (cerebrospinal fluid) side or the baso-lateral (blood) side of the epithelial cell. The putative serotonin active transport system in choroid plexus will be examined to determine if it bears any relation to the choroid plexus serotonergic binding site. Finally, the choroid plexus site will be solubilized and initial attempts made to purify and characterize it. These studies should contribute to our understanding of serotonergic sites in the brain and to our understanding of the roles of the choroid plexus in the blood-brain barrier and in cerebrospinal fluid production.